Incorporating a suitable window shade over a window can make a substantial difference in the aesthetics, comfort and energy savings in an enclosed space. In that regard, window shades are often utilized for a variety of purposes, for example maximizing a view, maximizing daylight, blacking out a room, minimizing brightness, adjusting to climatic variables and sky conditions, minimizing heat at certain times of the year, maximizing heat during other times of the year, protecting work surfaces, minimizing glare, protecting people from direct sun, and/or the like.
When choosing the appropriate window shade system, lighting designers and interior architects typically consider various factors, for example glazing (glass) properties, room properties and environmental conditions. The glass properties may include total solar and visible properties (e.g., transmission, reflection, absorption, and/or the like), single or multiple pane glass, chemicals or materials between the panes of glass, angled glass, tint, sun screens, ultraviolet (UV) transmission, bars over the windows, frosted glass, and/or the like. The room properties may include the interior lighting and the reflectance from the wall, floor and ceiling. The environmental conditions may include typical solar or climate conditions (e.g., often cloudy skies in Seattle, often clear skies in Phoenix, etc.), obstructions (mountains, trees, other buildings, and/or the like), luminance (the amount of light leaving a point on a surface in a given direction (e.g., that comes to the eye from a surface)) and illuminance (the amount of visible light on a surface from all directions above that surface or the density of luminous flux incident on a surface). Luminance may be measured in foot-lamberts, candala per square meter, nits, lambert, or any other suitable unit. Illuminance may be measured in lux, foot-candles, lumens per square meter, or any other suitable unit. The impact of a window shade system on a particular room may also be calculated under various conditions. For example, measurements may be obtained related to the luminance from the shade, walls and ceiling, the amount of light through the shade and glass, and/or the like.
In recent years, corporate and institutional building design has tended to include higher and higher visual light transmission glass the allowing more natural daylight into the building space, enhancing the view to the outside and using the daylight to reduce artificial lighting and A/C energy usage. Such increasing use of higher visual light transmission glass creates both problems and opportunities.
Since around the year 2000, designers have changed their selection of building glazings to low emissivity (“low E”) clear glass, low iron glass (e.g., Starphire® brand glass), or a similar tinted low E glazing. Such glazings have higher ratio of visible light transmission (VLT) to Solar Heat Gain Factor (SHGF). Over the years, the VLT of double glazing has changed from a low of about 0.20 (e.g. low E Solarban® brand glass), to an uncoated bronze or gray heat absorbing glass of about 0.40 VLT, to low iron glass (e.g., green or blue aqua glass) with a VLT of about 0.6, and now to a low F clear glass (e.g., Starphire® brand) glass with a VLT of about 0.70 to about 0.75. The SHGF percentage of heat inside the glass with a low E coating has remained around 0.40 to about 0.55. In other words, the ratio of heat gain to VLT through the glass was previously close to a 1:1 ratio, but the ratio is now about 1:1.75, which is a dramatic increase of VLT over heat gain.
Based in part on the lower heat gain resulting from improved glass, HVAC systems have been downsized. However, certain HVAC systems have sometimes not been sufficiently adjusted for the substantial gain in VLT, which also has a strong direct solar radiant component.
Moreover, in an effort to reduce glare and limit the impact of transient adaptation of the eye as the eye goes from one area (the task) to another area (adjacent surroundings or surfaces), lighting designers have determined an appropriate ratio for the perceived and measured brightness inside a person's field of view. A person's field of view is generally considered to be about a 60 degree visual cone. As used herein, “adjacent” surfaces or surroundings are those within about a 30 degree visual cone. “Non-adjacent” or “remote” surfaces or surroundings are those within a visual cone from about 30 degrees to about 60 degrees. A common recommended ratio between a task and adjacent surroundings is about 3:1, a common recommended ratio between task and remote surroundings is about 10:1, and a common recommended ratio is about 40:1 for everything outside a 60 degree cone. These ratios are applicable for areas on the order of one steradian, so higher ratios may be recommended in small areas to add visual interest.
To provide an example of commonly recommended ratios, if a work surface has 50 foot-candles (FC) of illuminance, then the visual zone should not have any glare or brightness that exceeds a 10:1 ratio within remote surroundings, in other words not to exceed about 500 FC. In another example, in the case of a computer screen, if the lumen output of the screen is 200 candela per square meter (cd/m2), then the maximum amount of brightness in the person's field of view should not be more than 2000 cd/m2 within remote surroundings.
As the VLT of the glass increases, and as lighting designers attempt to control the brightness inside a person's field of view, the shade cloth color on the outside and/or inside of a building also has an effect on the building design, along with the uniformity and alignment of the window covering. As such, the impact of the window covering is now becoming an integral element in building design.
To calculate the heat flow through glass by convection and direct radiation, an ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) formula using the solar optical properties of the fabric may be used to calculate SHGF of the glass, along with the glass and shade combination. Commonly available performance data for a screen fabric are Total Solar Reflectance (TSr); Total Solar Transmission (TSt); and Total Solar Absorbtion (TSa), wherein the total of TSr+TSt+TSa=100%. Other performance data for a screen fabric may include Visible Light Transmission (VLT); UV Transmission (UVT), and Openness Factor (OF) (a factor which is related to the closeness of the weave of the fabric). However, VLT, UVT, and OF are not included in the ASHRAE formula, as they are not components of the total solar measurements used above to develop the SHGF. The SHGF provides information useful for calculating heat flow into a building to enable mechanical engineers to more effectively size the HVAC systems.
However, the SHOP does not address the comfort factors of direct solar radiation, or visual brightness near the window wall (for example, up to about 15-20 feet from the window wall). The engineering standards of the shading coefficient, solar heat gain factor, or solar factor generally do not include a valuation of comfort at the window wall for an occupant. As such, tests were conducted to determine the factors that affect personal comfort near a window wall with sun screens. The tests matched different types and kinds of glass with a variety of woven sun screen fabrics, and measured the total heat gain, solar radiation, heat gain and visible light transmission. The tests resulted in a method for determining a screen cloth's “personal comfort value” under reasonable interior environmental conditions with glazing of a specific VLT and/or SHGF.
While various factors, tests and calculations exist for determining an optimum window system, the SHGF and the personal comfort values still do not include the relative brightness (illuminance) of the fabric when it is solar lit, its effect on the interior environment and its impact on the viewers. A strong need exists to compare the surface brightness of different fabrics with a uniform light source to determine the relative brightness of one screen fabric to another screen fabric. A strong need also exists to determine the illuminance or brightness factor of a screen such that the brightness factor can help determine a suitable window shade fabric for a particular room, building or other enclosure.